Hearing aids are generally well-known in the art and in wide spread use. In a typical hearing aid, a microphone is used to pick up sound waves and convert that information into electrical signals. An audio amplifier magnifies the electrical signals within the frequencies of interest (20 Hz to 20 KHz), and then sends the amplified signals to a speaker located at the inner portion of the hearing aid. The speaker converts the electrical signals back into sound waves. In technical literature concerning hearing aids, speakers are often referred to as "receivers".
Many conventional hearing aids are relatively large devices that are quite visible to other persons. A recent trend has been to make the hearing aid as small as possible, and to place a portion of it inside the ear where it is not visible. There are several patents which disclose hearing aids that ostensibly fit within the external auditory canal. It must be noted that, even in such patented inventions disclosing "in-the-canal" hearing aids, a portion of the hearing aid is visible and noticeable to other persons because the speaker and the electronics are too large to fit within the external auditory canal. One exception is disclosed in U.S. Pat. No. 4,817,609 by Perkins, wherein the external auditory canal is surgically enlarged so that the disclosed hearing aid can fit deep inside the canal, thereby showing very little to outside observers. Such surgery is an extraordinary remedy that most human users would wish to avoid if a more satisfactory hearing aid were available.
Other U.S. Patents that disclose hearing aids which ostensibly fit within the external auditory canal do not depict the exact anatomy of the external auditory canal. The external auditory canal (external acoustic meatus) leads from the concha (the "bowl" of the ear) to the tympanic membrane (eardrum). The outer one-third of the canal is cartilaginous, and the inner two-thirds is bony. The canal is not straight, but in the horizontal plane (a Transverse Section--see FIG. 3A) it takes a sharp turn, approximately 90.degree., toward the rear, and then a milder turn back toward the front as the path is traced from the concha toward the tympanic membrane. The area containing these "S-shaped" turns is designated the sigmoid portion of the cartilaginous part of the external auditory canal. Hearing aids that are disclosed as "straight" in overall shape are just not able to be located within the external auditory canal. Three patents that disclose such hearing aids are U.S. Pat. No. 4,520,236, by Gauthier, No. 4,539,440, by Sciarra, and No. 4,706,778, by Topholm.
The Gauthier patent describes a hearing aid that snugly fits inside the external auditory canal, apparently including the bony part of the canal. The hearing aid appears (from the drawings) to extend the entire length of the auditory canal, virtually against the tympanic membrane; such a device would surely be very uncomfortable to wear. Additionally, the Gauthier patent discloses the use of an earmold that would contain the device. Unless the earmold was very flexible, it would be impossible to insert the hearing aid into its intended location inside the external auditory canal; a "straight" configuration needed to snugly fit into the inner (bony) part of the canal would not be able to be placed through the sigmoid portion of the external auditory canal.
The Sciarra patent describes a hearing aid that has an adjustable diameter, which can be expanded (enlarged) in order to fit snugly inside the external auditory canal. The patent does not disclose precisely where the hearing aid is to sit in the canal. Since the drawings illustrate a "straight" device, it obviously cannot be placed very far into the canal, because it would not be able to make it through the sigmoid portion of the external auditory canal.
The Topholm patent describes a hearing aid that has a hollow space at its innermost tip, which acts as a resonance chamber by enhancing the device's frequency response in the 1000 Hz to 5000 Hz range. The patent does not disclose the location in the external auditory canal wherein the hearing aid is to be placed, nor does it disclose the exact shape of the entire hearing aid. All that is disclosed is a general tubular shape of the innermost tip, and it appears to fit somewhere in the cartilaginous part of the external auditory canal.
Another U.S. patent which discloses a hearing aid that ostensibly fits in the external auditory canal is U.S. Pat. No. 4,937,876, by Biermans. This patent does not disclose where the hearing aid is to sit in the external auditory canal. The drawings disclose a device which has a "receiver" (speaker) near its innter tip, with such speaker aiming directly toward the tympanic membrane. It is clear, however, that the speaker is too large in diameter to fit through the sigmoid portion of the external auditory canal, and therefore, this invention merely fits into the exterior opening of the external auditory canal with the major portion of hearing aid sticking outside the area of the concha.
It is important to note that, in order to minimize distortion in sound energy transferred to the tympanic membrane, a hearing aid speaker should have a surface area equal or greater than the surface area of the tympanic membrane. Since the surface area of the tympanic membrane is at least as great as an oblique cross-section area of the external auditory canal (as can be seen in FIGS. 3A and 4A of the present invention), it is therefore, obvious that a miniature speaker whose face is pointed directly at the tympanic membrane (as in the Biermans patent) must be at least as large as the cross-section area of the external auditory canal. The inevitable conclusion is that such a speaker cannot possibly fit past the sigmoid portion of the cartilaginous part of the external auditory canal.
The above four patents attempt to disclose hearing aids that are to be located in the external auditory canal. It is clear, however, from their general shape and size that a major portion of each of these devices must stick out of the ear in a manner that would be visible to others. Either the device is too "straight" to fit past the sigmoid portion of the external auditory canal, and/or the electrical components (including a battery) must reside outside the sigmoid portion of the canal due to their large overall size. Hence, the need for a miniature hearing aid that is small enough and properly shaped to fit deep inside the external auditory canal (without requiring ear surgery) has not yet been met by the above patented devices.
An improvement in the art was disclosed in U.S. Pat. No. 4,870,688, by Voroba. The Voroba patent describes a modular hearing aid which is shaped (and sized) to partially fit in the external auditory canal such that a large portion of the device is hidden from view by an outside observer. A portion of the device extends into the inner portion of the canal past the sigmoid portion of the external auditory canal. As the Voroba patent discloses, it is desirable to have the hearing aid extend further into the external auditory canal since the closer the hearing aid is to the tympanic membrane (eardrum), the greater the effective sound output of the hearing aid. The Voroba hearing aid uses a number of "hard" components, having individual geometries which provide for the accommodation of anatomical variations in individual users. The collection of modular hard parts are at least partially enclosed and extended by a compliant covering. The covering of the inner portion of the Voroba hearing aid is made of soft (compliant) material, and it may penetrate up to 3/4 of the length of the external auditory canal, thereby increasing the effective gain of the hearing aid by 6 to 10 dB over conventional "in-the-canal" hearing aids.
It must be noted, however, that the Voroba invention does not place its speaker at the innermost portion of the device. The speaker is, instead, located further toward the outer portion of the device (approximately in the center of the device according to the drawings), and a sound-carrying tube, surrounded by soft, resilient material, extends to the innermost tip of the device. In effect, the speaker (called a "receiver" in the Voroba patent) emits sound waves into the tube, and the tube acts as a passive wave guide toward the inner portion of the external auditory canal, and toward the tympanic membrane. The Voroba patent, therefore, only teaches the concept used in the prior art of having passive elements in the innermost portion of the hearing aid. Such passive elements are merely space-consuming conduits which transfer the acoustic energy from the active, sound-generating surface of the speaker. The air inside such passive element is compressible, so this system still lacks a certain amount of efficiency, and compromises the faithful reproduction of the soundwave at the tympanic membrane. In essence, the overall system of hearing aid speaker to tympanic membrane is not "closely-coupled."
Close coupling of an acoustic source to the tympanic membrane is necessary for the realization of the beneficial attributes gleaned by signal processing for the treatment of hearing deficit. Devices in the prior art for generalized signal processing, including U.S. Pat. No. 4,637,402 by Adelman, and U.S. Pat. Nos. 4,882,762, and 4,882,761 by Waldhauer, demonstrate optimization techniques for manipulating the electronic representation of the audio signal, but fail to provide optimal presentation as a sound wave to the tympanic membrane. Thus, generalized signal processing techniques of the prior art are limited by the ability of the output transducing device (the speaker) and, therefore, are not closely coupled systems.
To achieve a more closely-coupled system, the amount of compliant material between the active face of the speaker and the receptive face of the tympanic membrane must be kept to a minimum. The best method to achieve such a system is to reduce the volume of air (thereby reducing the amount of compliant material) contained in the active path of the sound waves. The beneficial effects of such a system are (1) better bandwidth, (2) greater efficiency of energy transmission, and (3) reduced distortion of the auditory signal. A better method for achieving such a closely-coupled system is to locate the active speaker itself inside the external auditory canal, as close to the eardrum as feasible, while also keeping the amount of compliant material (the amount of air volume) in the system to a minimum.